1. Field of the Invention
The present invention relates to a ceramic filter module for a fluid permeation filter, and more particularly to a silicon nitride filter module having high permeability.
2. Description of the Prior Art
Organic membranes have hitherto been used as a filter in the field of the production of foods, chemicals and semiconductors. Since, however, the organic membranes have poor heat resistance, pressure resistance, and chemical resistance, filters made of porous ceramic membranes which are excellent in the above properties have been substituted for the organic membranes. The porous ceramic membranes have also been used as a catalyst carrier, a bioreactor such as a microorganism cultivation carrier, and the like.
Further, in recent years, there is an ever-increasing demand for ceramic filters having high heat resistance, high strength, and high permeability. Among various ceramics, silicon nitride is a structural ceramic material having high strength, high toughness, high thermal shock resistance, and high chemical resistance and hence is a very promising filter material.
The efficiency of filtering of the ceramic filter is proportionally related to the porosity at constant pore diameter and inversely related to the pore diameter at constant porosity.
A conventional ceramic filter typified by Al.sub.2 O.sub.3 comprises a porous ceramic body prepared by sintering a raw material powder and hence has a low porosity, that is, a porosity of about 40% by volume at the highest. This leads to high pressure loss at the time of filtration, and the permeability is inferior to that of the organic membrane.
In order to eliminate the above drawback, an actual ceramic filter is designed so that a porous ceramic body is prepared in a tubular form having a multi-layer structure to reduce the thickness of the porous body in its small-diameter pore portions necessary for actual filtration, thereby lowering the pressure loss, thus increasing the permeability. More specifically, the ceramic filter has a multi-layer structure composed of two or three layers comprising a tubular thin layer section involved in actual filtration, a substrate section for supporting the thin layer section, and optionally an intermediate section interposed between the thin layer section and the substrate section.
When use of the ceramic filter on a commercial scale is contemplated, the membrane area per unit volume should be increased in order to render filtration equipment compact. This is because the performance of the filter is proportional to the product of the flow rate of permeation per unit area and the membrane area. The form of a filter developed for this purpose is a monolithic form. As shown in FIG. 8, this filter is in the form of a filter module 1 having a structure in a lotus root form in section wherein a large number of passage holes 3 for a feed fluid are provided in a porous ceramic body 2. Also in this monolithic form, as with the above filter, in order to increase the permeability, each passage hole 3 (referred to also as cell) has a multi-layer structure comprising a thin layer section, which has a small pore diameter and serves as a-filtration layer, and a substrate section having a large pore diameter for supporting the thin layer section.
The above monolithic filter module 1 has been extensively used for cross-flow filtration generally used in the production processes for foods and chemicals. In this filtration system, as shown in FIG. 9, a feed fluid 5 is poured from a stock solution tank 4 through a feed pump 6 into a monolithic filter module 1, and the permeate is withdrawn and recovered from the filter module 1, while the feed fluid 5 passing through the passage holes of the filter module 1 is returned through a circulation path 7 to the stock solution tank 4 and repeatedly flowed for filtration.
As described above, the conventional ceramic filter is in a tubular form having a multi-layer structure comprising a cylindrical thin layer section involved in filtration and a substrate section for supporting the thin layer section in order to lower the pressure loss and improve the permeability. In actual filtration equipment, use is made of a monolithic filter module having a multi-layer structure wherein the passage holes are formed in a lotus root form.
Since, however, a multi-layer structure of the lotus root-like passage holes in the monolithic filter cannot be prepared without resort to complicated steps, the production cost is inevitably increased unfavorably.
Meanwhile, a porous silicon nitride ceramic body having high porosity, high strength, and very high flow rate of permeation has been proposed in WO 94/27929, published Dec. 8, 1994. The porous silicon nitride body has a structure wherein columnar Si.sub.3 N.sub.4 particles as the main component are three-dimensionally and randomly bonded to one another. The porous body has a porosity of at least 30% and, unlike the conventional ceramic filter of Al.sub.2 O.sub.3 or the like, satisfactorily large flow rate of permeation can be provided without adopting a multi-layer structure.
Even in this porous silicon nitride ceramic body having a single-layer structure, use thereof as a monolithic filter module results in markedly lowered permeability, although the porous filter in the form of a tubular filter has higher permeability than the conventional Al.sub.2 O.sub.3 filter with a multi-layer structure.
The reason for this is as follows. In the monolithic filter having a single-layer structure, when the permeate passes from passage holes 3 near the center of the filter toward the circumferential surface through the ceramic body as indicated by an arrow in FIG. 8, the distances between the passage holes 3 near the center of the filter and the circumferential surface of the filter are large. This results in very large permeation resistance, and the actual permeability is substantially determined by the flow rate of permeation from the passage holes 3 near the surface of the filter.